1. Field of Invention
This invention is directed to systems and methods for fusing toner on image receiving mediums in image forming devices.
2. Description of Related Art
A variety of systems and methods are conventionally used to fuse toner particles on a variety of image bearing surfaces of image receiving mediums in image forming devices. Generally, such systems and methods involve a heated pressing surface usable to melt and press dry toner, which has been deposited on the image bearing surface of the image receiving medium in a preceding step of forming an image, onto or into the image bearing surface, thereby fusing the image on the image receiving medium.
Typical conventional systems and methods for fusing dry toner onto or into an image bearing surface of an image receiving medium in an image forming device include, for example, using what is commonly referred to as a hot roll fusing device to provide the heated pressing surface usable to melt and press the toner onto or into the image bearing surface of the image receiving medium. Such a hot roll fusing device 100 is shown, in exemplary manner, in FIG. 1. As shown in FIG. 1, the hot roll fusing device 100 includes a heated fuser roller 110 and an opposing pressure roller 120, the two rollers 110, 120 rotating respectively in directions depicted as arrows 115 and 125. The heated fuser roller 110 is formed of a hollow tube with a cylindrical surface 130 which may be closed at the ends with end caps 135 which may have vent holes as depicted. The heated fuser roller 110 contains a heat source in the form of a heating element 140.
Typical heating elements 140 usable to heat the cylindrical surface 130 of the heated fuser roller 110 are, for example, quartz rods or lamps. The cylindrical surface 130 of the heated fuser roller 110 may be configured of a hard metal or other rigid highly heat conductive substance and may optionally include an external heat-conductive coating such as, for example, Teflon® or the like. An objective of the construction of the heated fuser roller 110 is to conduct heat generated in the heating element 140 through the cylindrical surface 130 of the heated fuser roller 110 to the receiving medium 200 in order that toner, previously deposited on the image bearing surface of the image receiving medium 200, can be melted and pressed and therefore fused thereon.
Consistent, controllable heating along the entire axial length of a heated area of the heated fuser roller 110 is desirable in order to avoid any degradation in image quality due to insufficient, varied or excessive heating of the dry toner on the image bearing surface of the image receiving medium.
In order to further facilitate the fusing process in the hot roll fusing device 100, the second element of pressure is added, to supplement the first element of heat applied through the cylindrical surface 130 of the heated fuser roller 110, with inclusion of the pressure roller 120 that opposes the heated fuser roller 110. The pressure roller 120 is typically configured with a soft outer layer of, for example, silicon, rubber, or a silicon rubber shell. The pressure roller 120 presses firmly against the heated fuser roller 110. A softer surface on the pressure roller 120 provides additional surface-to-surface contact between the pressure roller 120 and the heated fuser roller 110 as the softer surface of the pressure roller 120 limitedly conforms itself to more of the heated cylindrical surface 130 of the heated fuser roller 110 than simply at a single line tangential to each roller where two hard rollers would interact. This limited conforming of the heated and pressure roller surfaces results in an increased exposure of any given point on the receiving medium 200 to the heat and pressure generated in the hot roll fusing device 100 by the contact on either side of the receiving medium 200 of the pressure roller 120 and the opposing heated fuser roller 110. The pressure, spread over more than a single tangential line between the two rollers 110, 120, facilitates heating and pressing of the toner to melt and press, and thereby fuse the toner onto or into the image bearing surface of the image receiving medium 200.
Such conventional systems thus facilitate fusing of toner on image receiving mediums 200 by melting the toner and simultaneously forcing the melted toner into the image bearing surface of the image receiving mediums 200. The softer the pressure roller 120, within certain limits, the better the surface of the pressure roller 120 conforms to the heated surface 130 of the heated fuser roller 110 forming the slightly larger contact area between the two rollers 110, 120. This contact area is commonly referred to as the nip.
In order to promote the highest quality image production and/or reproduction in image forming devices employing heat and pressure to fuse toner on an image receiving medium, it is desirable to provide controllable, consistent heat across an entire axial length of the cylindrical surface 130 of the heated fuser roller 110 with an objective of consistent heat and pressure on the image bearing surface of the image receiving medium 200. It is further desirable to control the temperature of the heated fuser roller 110 in a range that ensures that enough heat is supplied to melt the toner and thereby fuse the toner onto or into the image bearing surface of the image receiving medium 200, but that also ensures that not so much heat is supplied that image quality could be impaired, damage to the image or the image receiving medium 200 could occur, or damage to the hot roll fusing device 100 in the image forming device could result.
Maintaining uniform axial temperature along the entire cylindrical surface 130 of the heated fuser roller 110 has proven difficult to achieve in traditional hot roll fusing devices 100 such as the one depicted in exemplary manner in FIG. 1. Failure to maintain uniformity in temperature along an entire cylindrical surface 130 of a heated fuser roller 110 can result in impaired image quality because ineffective melting of toner can occur at positions which are relatively cooler, and image distortion and/or image offset can occur at relatively hotter axial locations.
In an effort to overcome these shortfalls, various systems and methods are employed. Careful selection of the material from which the heated fuser roller 110 is formed in an attempt to produce a heated fuser roller 110 whose cylindrical surface 130 has high, yet consistent, heat conductivity limits the selection of materials from which such a heated fuser roller 110 can be manufactured. A further limitation on the selection of heat conductive materials from which the heated fuser roller 110 is formed is consideration of reducing the overall thermal mass of the heated fuser roller 110 in order to minimize warm-up time required to bring the heated fuser roller 110 to a sufficient operating temperature to ensure melting of the toner.
Alternatively, a method for dealing with non-uniformity in the temperatures, particularly in image forming devices which accommodate varying widths of image receiving mediums 200, is to provide multiple hot roll fusing devices 100, such as that shown in exemplary fashion in FIG. 1, of varying widths within a single image forming device, or to incorporate multiple heating elements 140 in a single hot roll fusing device 100 that may be selectively employed to heat certain portions of heated fuser roller. Such built-in redundancy, while effective in increasing image quality, has the disadvantage of increasing the complexity of the image forming device. Such increased complexity will tend, for example, to adversely impact cost of manufacture of image forming devices and/or costs associated with maintenance of the devices and/or costs associated with production of images from such devices.
U.S. patent application Ser. No. 10/749,284, by the same inventor and commonly assigned, which is incorporated herein by reference in its entirety, discloses an improved hot roll fusing station and hot roll fusing method. An objective of the invention disclosed in the '284 application is to overcome at least the disadvantages enumerated above by providing a heated fuser member possessing consistent, controllable heating along an entire axial length. This objective is achieved by introducing electrical coils which are substantially co-axial to the heated fuser member and are positioned around or near at least one end of the heated fuser member and that are otherwise suitable for inductive coupling with the fuser member to inductively heat the fuser member when the electrical coil is energized with electrical power. The '284 application attempts to ensure consistent axial heating through the use of a heat pipe, i.e., a sealed hollow cavity which contains a working fluid. Such working fluid is optionally water, methanol or a combination of those, or another suitable working fluid in a multi-phase mixture with a liquid phase and corresponding vapor phase maintained in equilibrium. Employing such a working fluid, while better providing consistent axial heating, increases the complexity and, potentially, the serviceability of the disclosed heated fuser member, such as, for example, by requiring equipping the heated fuser member with a pressure relief system in order to protect against overpressurization.
The systems and methods disclosed in the '284 application realize a broadening of the options regarding the materials from which a heated fuser roller or heat pipe can be manufactured. For example, electrically conductive magnetic walled tube materials and electrically conductive non-magnetic walled tube materials are disclosed. It should be noted that typical heat pipes using water as a working fluid are constructed of copper or cupro-nickel materials. These materials, while exhibiting high electrical conductivity, may not be the most suitable for supporting induction heating.
Additionally, typically an over-temperature sensor is provided which is usable to induce an automatic slow down or cool down period or to ultimately interrupt processing within the image forming device completely. The objective of such a over-temperature sensor is to limit the possibility of damage to the image, the image receiving medium, or the image forming device based primarily on an uncontrolled or abnormal overheat condition in the hot roll fusing device.
U.S. Pat. No. 6,021,303 to Terada et al., the disclosure of which is incorporated herein by reference in its entirety, teaches generally an image heating device including a cylindrical heating roller and an internal magnetization coil for magnetizing the heating roller with an alternating magnetic field. This patent introduces a concept where, by selecting a Curie temperature for the magnetizing member, the heating roller temperature is self-regulated to stabilize at a temperature that is suitable for fusing toner on the image bearing surface of an image receiving medium. Specifically, a single or multiple layer heating roller that includes, as the single layer, or as one of the multiple layers, covering an entire span-wise heatable area of the fuser roller, a magnetic alloy. In an effort to ensure consistent heating across the entire span-wise heated area of the disclosed heating roller or member, the magnetization coil, located inside the heating roller, covers an entire span-wise heated portion of the heating roller.